The present invention relates generally to broadband communication networks such as local area networks (LANs) and metropolitan area networks (MANs) and particularly concerns a technique for providing equalized user access to and increased throughput in such networks.
Broadband LANs and MANs typically employ a two-waY cable television (CATV) plant to facilitate data communication between the user stations connected to the plant. Such data communication is effected by a source station transmitting an upstream data packet to the CATV headend which then retransmits the data packet downstream for reception by the desired receiving station. The upstream transmission from the source station is generally accomplished by means of an upstream channel (sometimes referred to as the reverse channel) typically having a carrier frequency ranging from 5 MHz up to 35 MHz in sub-split CATV systems and up to 174 MHz in high-split system configurations. The CATV headend includes a network translator which converts the reverse channel carrier to a downstream channel (e.g., a 234 MHz carrier), also referred to as the forward channel, and re-transmits the data packet on this channel.
The transmitted data packet originating at the source station includes an address code identifying the desired receiving station such that, upon re-transmission over the forward channel by the headend network translator, only the station identified by the address code will receive and decode the data packet. Also, the data packet is suitably coded to enable reception thereof by the source station which compares the received data packet with the transmitted data packet to verify its integrity.
In order to improve the performance of such broadband networks, it has been proposed to use a protocol commonly used in baseband LANs; namely carrier-sense multiple-access with collision detection (CSMA/CD). In a network using CSMA/CD, a source station will defer transmission of a formulated data packet until the forward channel is clear (i.e., no carrier present on the channel). Even so, two or more source stations may transmit data packets at substantially the same time resulting in packet collisions which destroy the information content of the transmitted message. Detection of such data packet collisions may be accomplished by source stations in a number of ways, e.g., bit-bit comparisons, CRC comparisons, code violations, etc., and in response to which the source stations abandons the remainder of the transmission. Subsequently, the source station may re-transmit the packet after a random delay so as to reduce the probability of further collisions.
In networks of the type described above, it will be appreciated that the stations physically closer to the headend network translator will have an advantage in terms of access to the system. In particular, due to the propagation time of a packet over either the forward or reverse channel, stations physically further from the headend network translator will have a reduced probability of capturing the channel than closer-in stations thereby reducing their access to the system as well as the overall system throughput. Neither the CSMA/CD protocol nor the prior art technique of independently calculating a random delay at each source station before retransmitting a packet solves this unequal system access problem, and the reduced system throughput associated therewith, since packet propagation times are not taken into account.